You can share segments in AA to experience cloud, those will come to AAM. If you want to do that from scratch in AAM with some additional criteria, then following are the instructions:
In AAM you will have to:
1. Create a data source of type cross device if those are customer IDs that you would like to ingest. 2. Define onboarded traits linked to that data source. 3. Then create a file with those Customer IDs and trait expressions and upload it to your AAM instance's FTP inbound or S3 inbound bucket.
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We want to import Device-Ids (IDFAs, Advertiser Ids from Android) into AAM to create segments based on this. We get all device_ids from our customer from their users. Can we upload them and just using this method: Inbound Data File Contents: Syntax, Invalid Characters, Variables, and Examples ? Those users didn't have any contact with AAM before (e.g. on the Website or in an App). But it should be possible to upload them to specific Traits and directly use them for activation purposes, or? Device-Ids are directly usable for activation as you don't need to establish a cookie-sync before.